The present exemplary embodiments broadly relate to the detection of synchronized defects in printing systems. They find particular application with printing and measuring single separation test targets to extrapolate print appearance for disparate colors of interest. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
Image non-uniformity occurs in the output images of digital image rendering devices, such as copiers, scanners, and printers, for a variety of reasons. Even relatively small non-uniformities can give rise to visibly objectionable print defects. Print quality can be tracked over time via inline and/or or offline sensing to react when output degrades beyond predetermined specifications. Conventionally, test pages are printed periodically to ascertain quality at any given point in time. Since every test page has an associated cost, however, existing approaches are often required to trade-off between a number of test colors of interest (sampling resolution in printer gamut) and how often such colors are printed and assessed (sampling resolution in time). These trade-offs are also often constrained by many factors such as inline vs. offline, paper and/or toner consumption, IQ measurement cost, productivity interruption, etc.
In printing systems, physical alignments, component tolerances, wear and component age can influence the uniformity with which colorants, such as inks and toners, are laid down across the surface of print media. Due to the characteristics of print operations, some defects are synchronized while others are not. Streaks are classic examples of synchronized defects where the locations of light streaks are the same from one page to another. Streaks are one-dimensional image defects that generally run parallel to the process direction in the printed image. Banding on the other hand, is generally a non-synchronized defect wherein the locations of light and dark repeated bands may vary from one page to another due to phases. Bands are one-dimensional image defects that generally run perpendicular to the process direction in a printed image. In some instances, however, banding defects can be synchronized when they always start at a particular phase due to the synchronization mechanism, such as page-sync, in some products.
Streaks can arise from non-uniform responses of the subsystems of an image rendering device, such as a xerographic marking engine, and can be constant over time in that they appear in relatively the same location from print to print. Photoreceptor scratches, contamination of the charger wire, non-uniform LED imager output and Raster Output Scanner (ROS) spot size variations, and spatially varying pressure on a bias transfer roll are examples of subsystem defects which can give rise to rendered image streaking in a xerographic marking engine.
Various methods exist for correcting image quality defects, once they are detected. These include modification of the tone reproduction curves (TRCs) used in converting the original image data into machine-dependent image data. Automated systems have been developed for compensating for non-linearity's introduced by an individual image rendering device. However, even with such techniques available, image rendering devices can produce images which have noticeable defects. When a print job is to be printed, a customer may test print several pages to ensure that the device chosen for rendering is performing satisfactorily. This process can be time consuming and also wastes supplies if the customer decides to check several image rendering devices before printing the job.
What are needed are methods to efficiently identify synchronized defects without negatively affecting throughput of a given device.